Fiber optic technology is being used to an increasing extent in the communications industry to carry various types of signals and data that were previously transmitted by means of ordinary metallic conductors and coaxial cables. In telephone communications, for example, the use of fiber optic lines between central offices and individual residential and business subscribers has allowed for large increases in signal traffic and improved signal quality. However, since the subscriber equipment is generally designed for connection to conventional metallic conductors, interface circuitry must be provided to allow signals to pass between the subscriber equipment and the optical fiber lines. This circuitry is generally housed in outdoor telephone cabinets placed at or near the subscriber locations, with a single cabinet typically serving a large number of individual subscribers.
In telephone cabinets of the type described above, racks or banks of circuits known as channel units carry out analog-to-digital and digital-to-analog conversion between the telephone company lines and the individual subscriber lines. Other circuits, known as optical-electrical interface (OEI) circuits, are provided for converting the digitized electrical signals to optical signals which can be transmitted on the fiber optic lines, and for converting received optical signals to digital electrical signals. Typically, both the channel units and the OEI circuits are provided in the form of slide-out cards which can be easily removed from the equipment rack when repair or replacement is needed.
Because optical fibers are manufactured in standard lengths with connectors already attached at both ends, it is generally necessary to use a fiber that is somewhat longer than needed when making connections to an OEI card or other optical interface device. The excess fiber length allows the equipment rack containing the OEI card to be removed and replaced at a different location in the telephone cabinet, should this become necessary. However, the excess fiber must be stored in some way, and this can present problems in the design of equipment racks and telephone cabinets. Because optical fibers are fragile and cannot be bent or deformed in the same manner that electrical wires can, it is difficult to achieve compact storage.
In cases where the optical fiber is coupled directly to the front panel of the OEI card, the excess fiber has sometimes been allowed to remain exposed at the front of the equipment rack. Unfortunately, the exposed fiber obscures the various printed indicia, visual indicators and circuit board extractors that are usually provided on the front panel of the equipment rack, and occupies space on the front panel that might otherwise be devoted to other hardware. The presence of the fiber on the front panel also requires that additional space be provided in the interior of the telephone cabinet, thereby requiring a somewhat deeper cabinet that might otherwise be desired. The exposed fiber is also susceptible to damage because of its fragility, and can expose repair personnel to potential eye injury from laser light if the fiber becomes inadvertently detached from the connectors on the OEI card. Finally, the exposed fiber is undesirable from an aesthetic point of view, since it creates an unsightly appearance in the interior of the telephone cabinet.
In order to avoid the problems referred to above, various arrangements have been proposed for storing the excess optical fiber at locations other than along the front panel of the equipment rack. In one arrangement, for example, the excess fiber is stored in pivoting drawers or trays which are recessed into the front panel of the equipment rack. Although this is advantageous in that it removes the excess optical fiber from the front panel area of the equipment rack, there is an additional degree of difficulty in that the drawers are usually grouped together at a location spaced from the OEI cards themselves, and hence the field repairman must take the time to locate and open the proper drawer in order to withdraw the corresponding OEI card. This can be inconvenient and time-consuming when the telephone cabinet contains a large number of OEI cards and optical fiber storage drawers.
Another problem that occurs in removing OEI cards from telephone cabinets has to do with undesirable bending or flexing of the optical fibers attached to these cards. In some OEI card designs, the connectors which couple the optical fibers to the OEI card are located on the portion of the OEI card that is normally enclosed within the interior of the equipment rack, rather than being mounted in an exposed location on the front panel of the OEI card. The connectors are usually inclined in an upward and rearward direction, approximately the direction in which the optical fibers are routed after leaving the OEI card. Because of the partially upward connector orientation and the fact that the OEI cards are usually withdrawn by means of a horizontal sliding movement, the optical fibers are subjected to some degree of bending or flexing in the region near the connectors when the OEI card is removed. Repeated flexing can cause the optical fiber, which is by its nature very fragile, to become damaged or broken.
In summary, the prior art has failed to provide an arrangement for storing excess optical fiber which is convenient to use, occupies minimal space within the interior of an electrical cabinet, protects the fiber from damage or breakage, and does not interfere with the front panel of the equipment rack from either a functional or aesthetic point of view. The prior art has also failed to provide an arrangement which allows an OEI card to be removed from an equipment rack without causing undue bending or flexing of the optical fiber at its point of attachment to the OEI card.